The purpose of this study was to validate the Teacher's Assessment of Students' Aptitudes Form (TASAF), an instrument devised to guide teachers assessment of students' aptitudes through the observation of their classroom behaviors, i.e., to what extent were teachers able to detect levels of differential aptitudes in their students through guided observation. Since the TASAF was designed to tap the same set of aptitudes included in the Battery of Differential Aptitudes Test (BDAT), the BDAT was used as the criterion for the validation of the TASAF. Students enrolled in the eighth grade of 9 Brazilian public school located in Rio de Janeiro took part in the study. The group was composed of girls and boys ranging in age from 13 to 18 years. The subjects belonged mostly to the middle-lower class, as indicated by their parents' occupation. Data treated by the "G" index of agreement indicated statistically significant correlations between measures on the two instruments. Results pointed to a promising alternative approach to the traditional method of assessing students' aptitudes through paper-pencil tests. (RC)